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The cobalt-platinum catalyzed reductive carbonylation of methanol to acetaldehyde
Journal of Molecular Catalysis, 26 (1984)
145
145 - 148
Letter
The Cobalt-platinum Catalyzed Reductive Carbonylation of Methanol to Acetaldehyde
GUY R. STEINMETZ Research Laboratories, Eastman port, TN 37662 (U.S.A.)
Chemicals Division, Eastman Kodak
Company,
Kings-
(Received March 19, 1984; accepted April 6, 1984)
Interest in multimetallic catalysis lies in its potential to accomplish unique chemical transformations inaccessible through monometallic systems [ 1, 21. There are, however, only a few multimetallic homogeneous catalysts known with certainty in synthesis gas chemistry [3,4]. Herein is reported a bimetallic cobalt/platinum catalyst for the carbonylation of methanol to acetaldehyde where the role of platinum is to facilitate the reduction of cobalt salts to [Co(CO)J. Previous studies have shown that Group VIII metal salts, such as RhC1s.3Hz0, RuC1s*3Hz0 and IrC13*3Hz0, are not effective as promoters for the cobalt/lithium iodide catalyzed carbonylation of methanol to acetaldehyde [ 11. However, it has been found that the addition of less than a molar equivalent of PtClz to a Co(OAc),.4H,O/LiI catalyst increases the reaction rate to approximately three times that for the unpromoted catalyst (Table 1) [ 5]*. Figure 1 illustrates that the addition of incremental amounts of PtClz results in a corresponding linear increase in rate until a levelling effect is observed at Pt/Co molar ratios greater than l/2?. The platinum co-catalyst appears only to enhance the cobalt chemistry because its effect does not change the product selectivities appreciably (Table 1) [ 5,6] $. The function of platinum in this catalyst system has been studied by using a high-pressure infrared cell to monitor the reaction in situ. The infrared spectrum of the Co/LiI catalyst in methanol at 100 “C under 1000 psig of carbon monoxide shows no metal carbonyl bands. As the tempera*These carbonylation experiments were undertaken in a 300 ml Hastelloy C autoclave designed to operate in a rocking mode. Details of experimental procedures have been described previously [ 11. fThese carbonylation experiments were undertaken similarly except in a stirred 1800 ml Hastelloy B autoclave with samples taken at predetermined intervals from a sample tube. All experiments were conducted over a 10 - 15% conversion range. Although not illustrated, all product concentrations increased in a linear manner similar to that shown in Fig. 1 for acetaldehyde. SThe effect of platinum is independent of synthesis gas composition [ 6b]. 0304-5102/84/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
146 TABLE 1 Carbonylation
of methanol to acetaldehydes Liquid products (mol)b
Catalysts (g) CO(OAC)~*~H~O PtClzC
LiI
MeOH
Hz0
AcHd
Me1
MeOAc
EtOAc
0.20 0.20 0.20 0.20
1.0 9.0 1.0 9.0
3.08 2.84 3.02 2.11
0.46 0.44 0.67 1.12
0.36 0:28 0.51 0.96
0.017 0.001 0.009
0.026 0.051 0.032 0.106
0.001 0.010
0.20 0.20
Mol % AcHe 93 85 94 85
aReaction conditions: 4000 psig; 1:l CO/H,; 195 “C; 0.5 h reaction time; 3.5 mol methanol. bDimethyl ether was not measured. ‘PtClz or PtClz/LiI are inactive as catalysts. dTotal acetaldehyde plus dimethyl acetal. eMol % AcH = mol AcH/mol (AcH + MeOH + EtOAc).
0.50
r
I
0.0
1.0
I 2.0
I
1
3.0
4.0
MMOL PtCI, ADDED TO 4 MMOL Co(OAc),
1
. 4&O
Fig. 1. The effect of incremental additions of PtClz to CO(OAC)~‘~H~O on the rate of acetaldehyde formation. Reaction conditions: 4000 psig; 1:l CO/H,; 130 ‘C; 6 h reaction time; 15 mol methanol; 25.0 g LiI.
ture reaches 150 “C, however, a broad band at 1905 cm-’ for [Co(CO)J slowly appears. In contrast, the 1905(s) cm-’ band and an unknown band at 2070(w) cm-’ appear rapidly in the infrared spectrum of the Co/Pt/LiI catalyst in methanol at 100 “C under 1000 psig of carbon monoxide. This reduction of Co(II) to Co(-I) occurs under conditions as mild as 50 “C and 1000 psig of 1:l CO/H2 in the presence of platinum, whereas no [Co(CO)J
147
is observed in the absence of platinum under comparable conditions [ 7]*. Although nothing is revealed about the pathway by which the reduction occurs, these experiments suggest that the platinum catalyzes the reduction of Co(I1). Literature precedence suggests that Co2(CO)s is a key intermediate in the reduction of Co(I1) to Co(-I) [eqn. (l)] [8]. 2C02+ + CO/H 2 &
HZ Co2(CO)s + 4H+ e ~HCO(CO)~ +
2H+ + 2 [ Co(CO),]-
The mechanism by which Co(I1) is reduced to Co2(CO)s by platinum is unknown and will not be considered here. However, several pathways that exist for the formation of [Co(CO)J from Co2(CO)s are considered in Scheme 1. Evidence will be presented to rule out two of them. The base Path A
/&0~+[c0(c0)‘$]; Ireduction
co2+-
Pt
I
+ \
Co2(COh
HCo( CO) 4 e
H+ + [ Co(CO),J
Path B
HdPt
HCO(CO)~ e
H+ + [ Co(CO),]-
Path C
Scheme 1.
catalyzed disproportionation of Co2(CO)s (path A), which is known to . __ occur rapidly at ambient pressure and temperature, does not occur when a methanol/CH2C12 solution of lithium iodide and Co2(CO)s under -1000 psig of carbon monoxide is mixed [9]. Only the infrared bands for Co2(CO)s are observed, and heating the solution to 100 “C results in no change. [Co(CO)J is not observed until the autoclave is vented to atmospheric pressure. Because the base-catalyzed disproportionation of Co2(CO)s to [Co(CO)J does not operate under conditions where the addition of platinum has been shown to have an effect, path A is eliminated. The cobaltcatalyzed reaction (path B) also does not appear to be operating under the reaction conditions. Under 1000 psig of 1:l CO/H2 at 50 “C, where the Co/Pt/LiI catalyst has been found to rapidly reduce Co(I1) to [Co(CO)&, Co2(CO)s in the presence of LiI is only slowly reduced to [ Co(CO)J**. Thus the role of platinum is apparently to catalytically reduce Co(I1) directly to [Co(CO)& (path C), although the intermediacy of CO~(CO)~ under these reaction conditions has yet to be confirmed.
*Small amounts of acetaldehyde and methyl acetate are observed in the IR and have been detected by gas chromatography, representing less than 1% conversion. Similar results have been reported for the carbonylation of methanol to acetic acid [ 71. **Co2(CO)s in the presence of platinum under these conditions results in the rapid formation of [ Co(CO)4]-.
148
The ability of platinum to catalyze the reduction of Co(I1) to Co(-I) also explains why its rate enhancement remains constant for the duration of the reaction instead of levelling off as [Co(CO)J is formed. In acidic media, CO~(CO)~ is known to reoxidize to Co(I1). This reoxidation would lead to a reduction in the rate if the platinum were not present [eqn. (l)] [ 101. This explanation is also consistent with earlier statements that the characteristics of the Co/Pt/LiI catalyst were identical to those of cobalt alone except for a higher rate. The identity of the platinum co-catalyst in unknown. A mixture of PtC!lz and LiI in methanol at 1000 psig CO or 1:l CO/H2 shows a single metal carbonyl band at 2070 cm -l that is similar to those of known platinum iodocarbonyl complexes [ll]. Unfortunately, the instability of the complex has prohibited its isolation. In summary, the cobalt/platinum homologation catalyst appears to represent a true homogeneous bimetallic catalyst. The role of platinum is to catalytically reduce Co(I1) to [ Co(CO)J at temperatures and pressures unattainable with cobalt alone. The observed rate enhancement is consistent with increased levels of an active cobalt catalyst which leads to a higher yield. At present, there is no evidence to suggest that platinum cooperatively assists cobalt in any key reaction step in the reductive carbonylation of methanol to acetaldehyde. Identification of the platinum co-catalyst and its application to other carbonylation reactions are in progress. References 1 G. R. Steinmetz and T. H. Larkins, Organometallics, 2 (1983) 1879, and references therein. 2 D. A. Roberts and G. L. Geoffroy, in G. Wilkinson, F. G. A. Stone and E. W. Abel (eds.), Comprehensive Organometallic Chemistry, Pergamon, London, 1982, Chap. 40. 3 (a) N. H. Alemdaroglu, J. L. M. Penninger and E. Oltay, Monatsh. Chem., 107 (1976) 1153. (b) J. Azran and M. Orchin, Organometallics, 3 (1984) 197. (c) U.S. Pat. 4 253 987 (1981) to R. A. Fiato (Union Carbide). (d) J. Smidt, W. Hafner, R. Jira, J. Sedlmeier, R. Sieber, R. Ruttinger and H. Kojer, Angew. Chem., 71 (1959) 176. 4 M. E. Fakley and R. A. Head, Appl. Catal., 5 (1983) 3. 5 U.S. Pat. 4 389 532 (1983) to T. H. Larkins and G. R. Steinmetz (Eastman Kodak). 6 (a) Details on product selectivities can be found in ref. 5 and may be cross-indexed by B. Cornils, in J. Falbe (ed.), New Syntheses with Carbon Monoxide, Springer-Verlag, New York, 1980, and references therein. (b) G. R. Steinmetz, unpublished results. 7 U.S. Pat. 3 856 856 (1974) to K. Nozaki (Shell). 8 M. Orchin, Act. Chem. Res., 14 (1981) 259, and references therein. 9 (a) P. S. Braterman, B. S. Walker and T. H. Robertson, J. Chem. Sot., Chem. Commun., (1977) 651. (b) I. Wender, H. W. Sternberg and M. Orchin, J. Am. Chem. Sot., 74 (1952) 1216. IO U.S. Pat. 3 634291 (1972) to S. Usami, K. Nishimura, S. Koyama and S. Fukuski (Toa Nenryo). 11 (a) M. J. Cleare and W. P. Griffith, J. Chem. Sot. (A), (1970) 2788. (b) N. M. Boag, P. L. Goggin, R. J. Goodfellow and I. R. Herbert, J. Chem. Sot., Dalton Trans., (1983) 1101.
145
145 - 148
Letter
The Cobalt-platinum Catalyzed Reductive Carbonylation of Methanol to Acetaldehyde
GUY R. STEINMETZ Research Laboratories, Eastman port, TN 37662 (U.S.A.)
Chemicals Division, Eastman Kodak
Company,
Kings-
(Received March 19, 1984; accepted April 6, 1984)
Interest in multimetallic catalysis lies in its potential to accomplish unique chemical transformations inaccessible through monometallic systems [ 1, 21. There are, however, only a few multimetallic homogeneous catalysts known with certainty in synthesis gas chemistry [3,4]. Herein is reported a bimetallic cobalt/platinum catalyst for the carbonylation of methanol to acetaldehyde where the role of platinum is to facilitate the reduction of cobalt salts to [Co(CO)J. Previous studies have shown that Group VIII metal salts, such as RhC1s.3Hz0, RuC1s*3Hz0 and IrC13*3Hz0, are not effective as promoters for the cobalt/lithium iodide catalyzed carbonylation of methanol to acetaldehyde [ 11. However, it has been found that the addition of less than a molar equivalent of PtClz to a Co(OAc),.4H,O/LiI catalyst increases the reaction rate to approximately three times that for the unpromoted catalyst (Table 1) [ 5]*. Figure 1 illustrates that the addition of incremental amounts of PtClz results in a corresponding linear increase in rate until a levelling effect is observed at Pt/Co molar ratios greater than l/2?. The platinum co-catalyst appears only to enhance the cobalt chemistry because its effect does not change the product selectivities appreciably (Table 1) [ 5,6] $. The function of platinum in this catalyst system has been studied by using a high-pressure infrared cell to monitor the reaction in situ. The infrared spectrum of the Co/LiI catalyst in methanol at 100 “C under 1000 psig of carbon monoxide shows no metal carbonyl bands. As the tempera*These carbonylation experiments were undertaken in a 300 ml Hastelloy C autoclave designed to operate in a rocking mode. Details of experimental procedures have been described previously [ 11. fThese carbonylation experiments were undertaken similarly except in a stirred 1800 ml Hastelloy B autoclave with samples taken at predetermined intervals from a sample tube. All experiments were conducted over a 10 - 15% conversion range. Although not illustrated, all product concentrations increased in a linear manner similar to that shown in Fig. 1 for acetaldehyde. SThe effect of platinum is independent of synthesis gas composition [ 6b]. 0304-5102/84/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
146 TABLE 1 Carbonylation
of methanol to acetaldehydes Liquid products (mol)b
Catalysts (g) CO(OAC)~*~H~O PtClzC
LiI
MeOH
Hz0
AcHd
Me1
MeOAc
EtOAc
0.20 0.20 0.20 0.20
1.0 9.0 1.0 9.0
3.08 2.84 3.02 2.11
0.46 0.44 0.67 1.12
0.36 0:28 0.51 0.96
0.017 0.001 0.009
0.026 0.051 0.032 0.106
0.001 0.010
0.20 0.20
Mol % AcHe 93 85 94 85
aReaction conditions: 4000 psig; 1:l CO/H,; 195 “C; 0.5 h reaction time; 3.5 mol methanol. bDimethyl ether was not measured. ‘PtClz or PtClz/LiI are inactive as catalysts. dTotal acetaldehyde plus dimethyl acetal. eMol % AcH = mol AcH/mol (AcH + MeOH + EtOAc).
0.50
r
I
0.0
1.0
I 2.0
I
1
3.0
4.0
MMOL PtCI, ADDED TO 4 MMOL Co(OAc),
1
. 4&O
Fig. 1. The effect of incremental additions of PtClz to CO(OAC)~‘~H~O on the rate of acetaldehyde formation. Reaction conditions: 4000 psig; 1:l CO/H,; 130 ‘C; 6 h reaction time; 15 mol methanol; 25.0 g LiI.
ture reaches 150 “C, however, a broad band at 1905 cm-’ for [Co(CO)J slowly appears. In contrast, the 1905(s) cm-’ band and an unknown band at 2070(w) cm-’ appear rapidly in the infrared spectrum of the Co/Pt/LiI catalyst in methanol at 100 “C under 1000 psig of carbon monoxide. This reduction of Co(II) to Co(-I) occurs under conditions as mild as 50 “C and 1000 psig of 1:l CO/H2 in the presence of platinum, whereas no [Co(CO)J
147
is observed in the absence of platinum under comparable conditions [ 7]*. Although nothing is revealed about the pathway by which the reduction occurs, these experiments suggest that the platinum catalyzes the reduction of Co(I1). Literature precedence suggests that Co2(CO)s is a key intermediate in the reduction of Co(I1) to Co(-I) [eqn. (l)] [8]. 2C02+ + CO/H 2 &
HZ Co2(CO)s + 4H+ e ~HCO(CO)~ +
2H+ + 2 [ Co(CO),]-
The mechanism by which Co(I1) is reduced to Co2(CO)s by platinum is unknown and will not be considered here. However, several pathways that exist for the formation of [Co(CO)J from Co2(CO)s are considered in Scheme 1. Evidence will be presented to rule out two of them. The base Path A
/&0~+[c0(c0)‘$]; Ireduction
co2+-
Pt
I
+ \
Co2(COh
HCo( CO) 4 e
H+ + [ Co(CO),J
Path B
HdPt
HCO(CO)~ e
H+ + [ Co(CO),]-
Path C
Scheme 1.
catalyzed disproportionation of Co2(CO)s (path A), which is known to . __ occur rapidly at ambient pressure and temperature, does not occur when a methanol/CH2C12 solution of lithium iodide and Co2(CO)s under -1000 psig of carbon monoxide is mixed [9]. Only the infrared bands for Co2(CO)s are observed, and heating the solution to 100 “C results in no change. [Co(CO)J is not observed until the autoclave is vented to atmospheric pressure. Because the base-catalyzed disproportionation of Co2(CO)s to [Co(CO)J does not operate under conditions where the addition of platinum has been shown to have an effect, path A is eliminated. The cobaltcatalyzed reaction (path B) also does not appear to be operating under the reaction conditions. Under 1000 psig of 1:l CO/H2 at 50 “C, where the Co/Pt/LiI catalyst has been found to rapidly reduce Co(I1) to [Co(CO)&, Co2(CO)s in the presence of LiI is only slowly reduced to [ Co(CO)J**. Thus the role of platinum is apparently to catalytically reduce Co(I1) directly to [Co(CO)& (path C), although the intermediacy of CO~(CO)~ under these reaction conditions has yet to be confirmed.
*Small amounts of acetaldehyde and methyl acetate are observed in the IR and have been detected by gas chromatography, representing less than 1% conversion. Similar results have been reported for the carbonylation of methanol to acetic acid [ 71. **Co2(CO)s in the presence of platinum under these conditions results in the rapid formation of [ Co(CO)4]-.
148
The ability of platinum to catalyze the reduction of Co(I1) to Co(-I) also explains why its rate enhancement remains constant for the duration of the reaction instead of levelling off as [Co(CO)J is formed. In acidic media, CO~(CO)~ is known to reoxidize to Co(I1). This reoxidation would lead to a reduction in the rate if the platinum were not present [eqn. (l)] [ 101. This explanation is also consistent with earlier statements that the characteristics of the Co/Pt/LiI catalyst were identical to those of cobalt alone except for a higher rate. The identity of the platinum co-catalyst in unknown. A mixture of PtC!lz and LiI in methanol at 1000 psig CO or 1:l CO/H2 shows a single metal carbonyl band at 2070 cm -l that is similar to those of known platinum iodocarbonyl complexes [ll]. Unfortunately, the instability of the complex has prohibited its isolation. In summary, the cobalt/platinum homologation catalyst appears to represent a true homogeneous bimetallic catalyst. The role of platinum is to catalytically reduce Co(I1) to [ Co(CO)J at temperatures and pressures unattainable with cobalt alone. The observed rate enhancement is consistent with increased levels of an active cobalt catalyst which leads to a higher yield. At present, there is no evidence to suggest that platinum cooperatively assists cobalt in any key reaction step in the reductive carbonylation of methanol to acetaldehyde. Identification of the platinum co-catalyst and its application to other carbonylation reactions are in progress. References 1 G. R. Steinmetz and T. H. Larkins, Organometallics, 2 (1983) 1879, and references therein. 2 D. A. Roberts and G. L. Geoffroy, in G. Wilkinson, F. G. A. Stone and E. W. Abel (eds.), Comprehensive Organometallic Chemistry, Pergamon, London, 1982, Chap. 40. 3 (a) N. H. Alemdaroglu, J. L. M. Penninger and E. Oltay, Monatsh. Chem., 107 (1976) 1153. (b) J. Azran and M. Orchin, Organometallics, 3 (1984) 197. (c) U.S. Pat. 4 253 987 (1981) to R. A. Fiato (Union Carbide). (d) J. Smidt, W. Hafner, R. Jira, J. Sedlmeier, R. Sieber, R. Ruttinger and H. Kojer, Angew. Chem., 71 (1959) 176. 4 M. E. Fakley and R. A. Head, Appl. Catal., 5 (1983) 3. 5 U.S. Pat. 4 389 532 (1983) to T. H. Larkins and G. R. Steinmetz (Eastman Kodak). 6 (a) Details on product selectivities can be found in ref. 5 and may be cross-indexed by B. Cornils, in J. Falbe (ed.), New Syntheses with Carbon Monoxide, Springer-Verlag, New York, 1980, and references therein. (b) G. R. Steinmetz, unpublished results. 7 U.S. Pat. 3 856 856 (1974) to K. Nozaki (Shell). 8 M. Orchin, Act. Chem. Res., 14 (1981) 259, and references therein. 9 (a) P. S. Braterman, B. S. Walker and T. H. Robertson, J. Chem. Sot., Chem. Commun., (1977) 651. (b) I. Wender, H. W. Sternberg and M. Orchin, J. Am. Chem. Sot., 74 (1952) 1216. IO U.S. Pat. 3 634291 (1972) to S. Usami, K. Nishimura, S. Koyama and S. Fukuski (Toa Nenryo). 11 (a) M. J. Cleare and W. P. Griffith, J. Chem. Sot. (A), (1970) 2788. (b) N. M. Boag, P. L. Goggin, R. J. Goodfellow and I. R. Herbert, J. Chem. Sot., Dalton Trans., (1983) 1101.